Field of the Invention
The present invention relates to a stand-alone microgrid, and more particularly, to an inverter-based stand-alone microgrid control system using a PMU that enables each converter-based power supply to be accurately operated by conducting a grid analysis based on synchronization data acquired by the PMU and thus providing an accurate operation point.
Description of the Related Art
Recently, along with emergence of a smart grid, a microgrid power grid independent of an existing power grid has been widely employed in island areas or remote areas in Korea and abroad by using a distributed power supply such as wind power, solar energy, and the like, and an energy storage system. However, it is frequently in need of maintenance and repair due to its breakdowns and low reliability.
Microgrid power grids can be classified by a connection method and a control method.
The microgrid power grids can be classified by a connection method into an AC microgrid in which components are interconnected in an AC manner and a DC microgrid in which components are interconnected in a DC manner.
The AC microgrid uses a conventional distribution network but causes problems with synchronization, stability, and reactive power.
Meanwhile, the DC microgrid does not have any problems with synchronization, stability, and reactive power and does not need a two-step power conversion process when connecting power generated from each power supply, resulting in a low system loss and a low cost.
Particularly, most of digital loads sharply increased in use in recent years need DC power, and, thus, the DC microgrid is more efficient. Therefore, the DC microgrid has been at the center of a lot of attention recently.
The microgrid power grids can also be classified by a control method. Firstly, there is a method in which a central controller is provided and a system is operated by measuring energy of components in real time. According to this method, a sensor for measuring energy and a communication network for transmitting measured data to the central controller are needed.
This method has an advantage in that the central controller rapidly receives information of each of components using a high-speed communication network and controls them, and, thus, it is possible to readily operate a power grid, but also has a disadvantage in that it needs a climate prediction algorithm for air volume or amount of insulation in the case of using a distributed power supply such as wind power and solar energy and has a high level of dependence on communication.
Further, this method uses an operation algorithm depending on power transaction and focuses on the maintenance of power balance of a high-level grid. Thus, this method is suitable for a grid-interconnected microgrid.
As another control method, there is an autonomous control method in which converters respectively connected with components (distributed power supply, energy storage system, and the like) of a microgrid power grid autonomously control the components connected thereto.
According to this method, the respective components are configured to autonomously and somewhat independently control their own operations and readily operate a microgrid power grid overall.
According to this method, an expensive communication system is not required and a demand-side can be autonomously managed with a simple operation algorithm. However, since a state transmission cannot be made between devices, the energy storage system is overburdened, which may cause a sharp reduction in life span and may also cause a circulating current between the components, and makes it difficult to efficiently operate a distributed power supply.
However, this autonomous control method does not require a high-speed communication network and does not need to use a climate prediction algorithm and a complicated control algorithm. Thus, it is suitable for a stand-alone microgrid power grid which is operated separately from a grid.
Since the stand-alone microgrid is independently operated as being isolated from an existing power grid, the maintenance of power balance during an operation is the most important factor and determines reliability.
As such, if the autonomous control method is applied to the stand-alone microgrid, a high-speed communication network, a climate prediction algorithm, and a complicated central control algorithm are not needed. However, a circulating current may occur and a life span may be decreased due to excessive use of the energy storage system.
Therefore, if a stand-alone DC microgrid power grid is controlled based on the autonomous control method, a control method for improving reliability depending on power balance in a microgrid power grid, suppressing occurrence of a circulating current between power components, and improving a life span of an energy storage system is demanded.
If a plurality of converter-based distributed power supply devices is connected to a stand-alone microgrid, a power flow is determined depending on a relative voltage and a phase of each converter.
The power flow determines an output of each converter. Therefore, it is very important to determine a voltage and a phase at a point where each converter is connected. A conventional P-Q control method of directly controlling an output of each converter makes a grid unstable, and, thus, cannot be employed.